The present invention is directed to a resonant circuit, particularly for a monolithic integration in electronic circuits.
High-quality filters and oscillators having low phase noise, which in turn require resonant circuits with high quality, are required for constructing highly selective reception or, respectively, transmission circuits, for example for use in radio communication systems. Increasing miniaturization in electronics means that filters and oscillators must also be available in a suitable size, i.e. in the size of integrated circuits. According to the prior art, such high-quality resonant circuits are realized as discrete elements, for example in the form of surface wave filters or quartz resonators. A complete integratability of these elements in electronic circuits is not possible without further ado because of different substrate materials, as a result whereof these solutions have the disadvantage of a high space requirement and of an increased cost expenditure in manufacture. Electrical resonant circuits, by contrast, can be completely integrated but exhibit the disadvantage of having only low quality that, in particular, is not adequate for application in radio communication systems.
Thge prior art of H. Lobensommer, "Handbuch der modernen Funktechnik", Franzis-Verlag, Poing, 1995, pages 80 through 90, discloses monolithic quartz resonators and surface wave filters. What monolithic means, given quartz resonators, is that the function of a plurality of discrete elements (discrete resonators) is realized on one quartz substrate. The electrode pairs are arranged and dimensioned such on the substrate that the resonance can only form in limited zones. A coupling ridge that determines the respective coupling factor forms between the electrodes. The oscillatory amplitude decreases exponentially outside these zones. The discrete elements or discrete resonators are acoustically connected to one another via the oscillatory energy that is coupled out. Monolithic quartz resonators are manufactured for frequencies of into the range of approximately 100 MHZ. Surface wave filters are manufactured in such a way that a metal layer is vapor-deposited on a single-crystal, piezoelectric substrate and the structure and interdigital transducer is produces with a photo etching technology. Interdigital transducers are comb-like or, respectively, finger-like structures that engage into one another and whose fingers thereby overlap. These structures can comprise more than 100 fingers and respectively form a piezoelectric transducer that can generate and receive surface waves on the substrate.
The publications of T. W. Kenny et al in Appl. Phys. Lett. 58, 100 through 102 (1991) and in J. Vac. Sci. Technol. A 10 (4), 2114 through 2118 (1992) disclose tunnel effect acceleration sensors wherein self-supporting beams or rectangles resiliently anchored due to under-etching are applied on silicon. An excursion of the resilient part as a consequence of forces of inertia given accelerations can be detected with the sensors. These sensors are manufactured by under-etching at the surface of a compact silicon block. European reference EP 0 619 494 discloses a tunnel effect acceleration sensor wherein the manufacture ensues in process steps of micromechanics that are compatible with the technology for manufacturing integrated circuits. This method of manufacture has the advantage that a following electronic circuit can be integrated in the silicon substrate together with the sensor and precision and signal-to-noise ratio of the sensor can thus be significantly improved.